1. Field of the Invention
The present invention generally relates to fluid filtration. More particularly, the present invention relates to a filter assembly for high pressure, high flow rate and low pressure drop applications.
2. Discussion of the Related Art
Fluid cleanliness and viscosity are two important properties of hydraulic fluid in a fluid power system. Contaminants may be supplied to the hydraulic system from sources both internal and external to the system. The level of undesirable contaminants in the hydraulic fluid affects the quality of system performance, as well as the useful life of substantially all of the working hydraulic components within a hydraulic system. All moving components in contact with the fluid are vulnerable to wear, and attendant premature failure if such contaminants are not removed from the system. Consequently, proper cleaning of the fluid to remove undesirable contaminants can significantly lengthen the life of the system components, as well as reduce maintenance and its attendant costs. Further, effective cleanliness control can result in significant improvements in the overall reliability and performance of the system.
Maintenance of a clean hydraulic fluid requires efficient filtration. A number of methods have been utilized to control the cleanliness of the fluid in hydraulic systems. The filters utilized in typical cleanliness control systems must withstand high pressure and/or high volume flow in certain applications. Consequently, such filter arrangements are often expensive and can contribute to related system problems.
Higher demands are made upon the hydraulic systems of aircraft. Microscopic particles present a significant problem because it is difficult to manufacture a filter element that is capable of removing very small particles and at the same time has a sufficient flow capacity and low pressure drop to meet the flow requirements of typical aircraft systems.
The flow capacity of a filter is a function of the surface area and micron removal rating. Aircraft have limited space and weight requirements. It is difficult to manufacture a filter element that is capable of removing fine particles, has a high flow capacity, a low pressure drop, is small in size, and is rugged enough for aircraft hydraulic systems.
For example, a filter may be interposed in line before the load to provide full flow filtering. This method is effective in many types of systems having relatively low fluid flow, e.g., 30 gallons per minute (gpm) or less. However, many hydraulic systems provide relatively large flows at high pressures, often running on the order of 400 gpm at pressures of 1000 pounds per square inch (psi) or greater. Interposing a filter in line before the load is often impractical in those high pressure systems with relatively large fluid flows. Further, maintaining filters in such an environment is generally quite expensive.
Alternately, full flow filtering may be provided after fluid has serviced the load. In this method of filtering, a filter is typically interposed in the return line between the load and the sump. Although less costly than filtering systems having the filter disposed before the load, return oil filtering can still be quite costly. Additionally, as return line filters become dirty, they develop back pressure. The development of back pressure can be a problem in that a number of valving systems do not perform properly with the application of back pressure.
An additional method of filtering disposes a filter in the sump. By nature, these filters are coarse so as not to affect flow of fluid to the pump. Consequently, while this method may be effective for filtering large particles, small particles are not effectively blocked.
Engine oil lubrication systems, which are typical of many fluid systems, frequently include a filter assembly which has a filter formed from a porous filter medium for removing damaging particles from the lubricating oil utilized in the system. Mechanical wear within the engine, the outside environment, and contaminants accidentally introduced during normal servicing provide a source of large particles which may plug lubricating nozzles or severely damage parts and create excessive wear on any surfaces relying on a thin film of the lubricating oil for protection.
These systems typically rely upon a pump to force the oil through the filter and then circulate the filtered oil to the moving parts of the engine for lubrication. Oil is forced through the filter by limited pressure developed on the upstream side of the filter by the oil pump. The pressure required to force oil to pass through the filter at a given rate will be greater for more viscous or thick oils or for filters formed from finer pored filter media, i.e., porous filter media having smaller average or mean pore diameters.
Viscosity is a measure of the resistance of the fluid to flow, or, in other words, the sluggishness with which the fluid moves. When the viscosity is low, the fluid is thin and has a low body; consequently, the fluid flows easily. Conversely, when the viscosity is high, the fluid is thick in appearance and has a high body; thus, the fluid flows with difficulty.
Oil is generally thicker or more viscous at low temperatures and thus, when an engine is started and the engine parts and oil are cold, a larger pressure is required to force the oil through the filter than after the engine has reached operating temperature. Since the pump frequently has limited pressure capabilities, many systems include a bypass valve, which will open when the pressure exceeds a predetermined value and allow oil to bypass the filter. This results in unfiltered oil being pumped through the engine where large particles may harm the moving parts and clog passages. Further, the high upstream pressure developed during a cold start may cause the lighting of a high pressure oil light, erroneously indicating that the filter is dirty or that the lubrication system is otherwise obstructed.
Automatic self-compensating flow control lubrication systems for continuously supplying the requisite amount of lubricant to at least one moving component of a drive system are known in the art. Various applications require that fluid condition in a mechanical system be continuously monitored and adjusted to maintain optimum overall system performance.
Present lubrication systems of the type used, for example, in drive systems for gas turbine engines are designed to supply a near constant oil pressure to fixed jets in the various engine components which require lubrication including bearing package, gears and the like. Systems such as this are designed to supply the minimum flow required for the worst case. This philosophy inevitably leads to excessive flow conditions in most other engine operating modes. Deteriorating system conditions, such as clogging jets, cannot be corrected and require operator attention with the possibility of mission cancellation.
In addition to the primary flow functions of the system, present configurations include some diagnostic and condition monitoring provisions. However, these are mainly warning lights and/or gages, which require crew attention and only add to the operator workload.
One such system is disclosed in U.S. Pat. No. 5,067,454 (“the Waddington et al. reference”). The disclosed invention relates to an automatic self compensating flow control lubrication system. One or more operating parameters, such as scavenge temperature, are continuously monitored and the information provided to a computer. The computer operates the first stage solenoid valve of a two stage valve assembly which provides such an amount of lubricant to the component as is necessary to maintain a predetermined value of the operating parameter. Scavenge temperature is one such operating parameter.
In the operation of this lubrication system, oil or other suitable liquid lubricant, is drawn from a reservoir by means of a suitable pump through a replaceable filter assembly which incorporates a controlled bypass valve which, together with the filter assembly is an integral part of the pump assembly. The bypass valve allows essentially dirty oil to be supplied to the components of the drive system requiring lubrication in emergency situations during which the filter is clogged. Alternatively, it operates to continue flow of oil during cold weather starting when the oil is too viscous to pass through the filter.
A computer controlling operation of the lubrication system controls whether and when the bypass valve opens. Other similar prior art systems open and close the bypass valve at fixed points, which have the effect of reducing filter life. The Waddington reference, by opening the bypass valve only when absolutely necessary, increases filter life and life of the drive system by reducing the time that dirty oil is supplied to the components requiring lubrication.
U.S. Pat. No. 4,783,271 (“the Silverwater reference”) discloses a filter assembly which removes particles from a fluid and which comprises two filters and a structure for directing the fluid first through one filter and then through the other. Each filter includes a porous filter medium. However, the filter medium of the downstream filter is coarser than the filter medium of the upstream filter, i.e., the mean pore diameter of the porous filter medium of the downstream filter is greater than the mean pore diameter of the porous filter medium of the upstream filter.
The filter assembly further includes a mechanism for sensing the temperature of the fluid and a valve, which is responsive to the temperature-sensing mechanism. The valve is arranged in parallel with the upstream filter so that, when the fluid temperature reaches a predetermined value as sensed by the sensing mechanism, the valve opens, allowing the fluid to bypass the upstream filter and flow through the coarser downstream filter. For example, in one embodiment of the invention, the valve is open when the fluid temperature is below the predetermined value.
With the filter assembly according to the Silverwater reference, the fluid is always filtered, regardless of the temperature of the fluid. When the fluid temperature increases, e.g., approaches the normal operating temperature, and reaches a predetermined value, as sensed by the sensing mechanism, the valve closes, causing all the fluid to flow through both filters. Thus, the finer upstream filter removes all particles from the fluid while the coarser downstream filter serves as a backup filter in case the upstream filter is damaged or defective.
However, when the temperature of the fluid, as sensed by the sensing mechanism, falls below the predetermined value, e.g., falls below a predetermined lower limit when the engine is shut down, the valve opens. Consequently, when the engine is next started, the fluid partially bypasses the upstream filter but all of the fluid is passed through the coarser downstream filter.
The downstream filter may frequently be physically smaller than the upstream filter. Therefore, in order to minimize the obstruction to flow by the downstream filter when filtering cold, viscous oil, the downstream filter preferably has a much larger mean pore diameter than the upstream filter. However, the mean pore diameter of the downstream filter is nonetheless small enough that the filtration provided by the downstream filter is sufficient to remove any large particles which may have been introduced into the fluid.
The size and the weight of a filter assembly are major factors in hydraulic system design, especially in aerospace applications. These demands, coupled with the further requirements of low pressure drop, high flow rates and improved fatigue life at continually increasing operating pressures, require departure from the standard design approach in hydraulic systems.
Therefore, there is a need for an innovative approach in the design of a high pressure hydraulic filter module, which provides both the required performance (low pressure drop at a high flow condition) and the structural integrity (1,000,000 impulse cycles from 0 to 6000 psi) and at the same time reducing the weight by as much as 50% from a conventional design.